This invention relates to a small, self-contained apparatus for the detection and ranging of lightning discharges. The device is sufficiently small to be carried in the hand and in the pockets of normal garments of apparel.
Previous developments in the field of lightning detection include ranging systems in which the distance to lightning strokes is determined (U.S. Pat. No. 3,715,660, Ruhnke) and more elaborate devices which display both azimuth and distance to lightning activity (U.S. Pat. No. 4,023,408, Ryan et al; U.S. Pat. No. 4,422,037, Coleman). These inventions typically employ cross-loop antennas to detect the magnetic field component of the radiation which emanates from lightning activity and nondirectional radio antennas to detect the electric component. Comparisons of the magnetic and electric field strengths then yield approximate distances to lightning strikes.
Some devices include components which perform waveform analysis in order to distinguish lightning activity from other natural or manmade electromagnetic radiation (U.S. Pat. No. 4,198,599, Krider et al).
Other systems employ principals of interferometry to locate lightning activity (U.S. Pat. No. 4,841,304, Richard et al) or the phenomena of induced corona discharge to detect the potential of lightning strikes (U.S. Pat. No. 4,823,228, Bittner).
Most such devices are conceived as fixed, land based installations or airborne instruments involving substantial size, weight and expense.
Those aforementioned devices that measure distances to lightning activity based upon comparisons of the electrostatic and magnetic field components emanating from lightning discharges rely upon a composite field equation: ##EQU1## where D=the distance to the electromagnetic disturbance and c=the speed of light.
The first term of the equation represents the electrostatic field component, the second term the inductive field, and the third term the radiation field component. Inspection of equation (1) reveals the first term to be the most sensitive indicator of distance between detector and source of lightning induced fields. Accordingly, the present invention focuses upon the detection of the electrostatic field as a means of determining distance to lightning strikes.
Given that peak current flow in lightning discharges has been estimated to vary between 7,000 and 28,000 amperes (Ryan, U.S. Pat. No. 4,023,408), and that fluctuations in signal strength need to be controlled for in order to more accurately estimate distance, the present invention selectively detects and analyzes a plurality of signals in the very low frequency range (1,400 hz and 700 hz in this instance) rather than relying merely upon quantitative comparisons of signal amplitude alone.
Ryan mentions that lightning strokes of greater than average peak current flow appear to be closer than they actually are as displayed on the Stormscope and strokes of lower than average peak current appear further away. This condition is referred to as "error" although it may be considered to be "useful approximation" if one's concern is merely to remain as far away as possible from any lightning activity.
The significance of distance measurement through low frequency signal detection becomes clearer if equation (1) is expressed as a function of frequency E(w) where w=observation frequency in radians per second (rad./sec.). ##EQU2## As in equation (1) the three terms correspond to the electrostatic, inductive and radiation fields, but it should be noted that the electrostatic component can be made to dominate at any distance by an appropriate selection of frequency. The operant equation for this purpose is EQU D.sub.c +c/w (3)
where D.sub.c =critical distance in miles and c=the speed of light in miles per second (miles/sec.).